The quick answer: for a normal Lisbon room, allow roughly 9,000 BTU up to ~15 m², 12,000 BTU to ~25 m², and 18,000 BTU to ~35 m² — then add capacity for sun, top floors, big windows and high ceilings. Getting it right matters: an undersized unit runs flat out and never quite cools, an oversized one short-cycles and wastes power. Here's the full rule of thumb.
Brought to you by the local team behind AC Rental Lisbon — years of keeping Lisbon homes and apartments comfortable.
A rough guide for a normal Lisbon room. Add capacity for the factors below — sun, top floors, glazing and high ceilings.
Rule of thumb only — we size precisely from your actual room in the free quote.
Floor area is the starting point, not the whole story. These factors push your BTU up.
A sun-facing room or big windows gain a lot of heat. Add roughly 10–20% for strong afternoon sun or large glazed areas — common in Lisbon's west- and south-facing flats.
Heat rises and roofs bake in summer. A top-floor flat or a room directly under the roof typically needs about 10–20% more capacity to keep up.
Lisbon's tall old rooms hold more air, so size by volume, not floor area. Open-plan spaces and heat from an open kitchen add to the load too.
See what it costs in our installation cost guide, compare single-split vs multi-split, or go to AC installation in Lisbon.
For a typical Lisbon bedroom up to about 15 m², a 9,000 BTU unit is usually right; up to around 25 m², step up to 9–12,000 BTU. Add capacity if the room faces the sun, is on the top floor or has large windows. A bedroom benefits from not oversizing, so it dehumidifies well and stays quiet.
A typical living room of 25–35 m² usually needs 12–18,000 BTU, and a large or open-plan space of 35–50 m² around 18–24,000 BTU. Open kitchens, big windows and high ceilings push it higher — which is why we size from the actual room rather than floor area alone.
A rough rule for Portugal is about 300–350 BTU per m² for a normal, reasonably shaded room — so 20 m² ≈ 6,000–7,000 BTU, rounded up to the nearest standard unit. Sun, top floors, glazing and high ceilings raise the figure, so treat per-m² numbers as a starting point, not a final answer.
Yes. A top-floor flat or a room under the roof gains extra heat, and strong afternoon sun or large windows add more — allow roughly 10–20% extra for each. Two of these together (say a sunny top-floor living room) can move you up a whole unit size.
No. An oversized unit cools the air quickly then switches off before it removes humidity, so it short-cycles, leaves the room clammy and wears out faster — as well as costing more to buy. The goal is the right size for the room, not the biggest.
It runs at full power constantly, struggles to reach the temperature you set on hot days, uses more electricity than a right-sized unit and wears out sooner. Undersizing to save a little upfront usually costs more in comfort and bills — sizing correctly is worth it.
Give us the room size, the floor, which way it faces and the ceiling height, and we'll tell you the right BTU and roughly what it costs — no guessing. Minutes to reply on WhatsApp.
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